This functionality is experimental and may be changed or removed completely in a future release. Elastic will take a best effort approach to fix any issues, but experimental features are not subject to the support SLA of official GA features.

While SQL and Elasticsearch have different terms for the way the data is organized (and different semantics), essentially their purpose is the same.

So let’s start from the bottom; these roughly are:

SQL

Elasticsearch

Description

column

field

In both cases, at the lowest level, data is stored in named entries, of a variety of data types, containing one value. SQL calls such an entry a column while Elasticsearch a field.
Notice that in Elasticsearch a field can contain multiple values of the same type (essentially a list) while in SQL, a column can contain exactly one value of said type.
Elasticsearch SQL will do its best to preserve the SQL semantic and, depending on the query, reject those that return fields with more than one value.

row

document

Columns and fields do not exist by themselves; they are part of a row or a document. The two have slightly different semantics: a row tends to be strict (and have more enforcements) while a document tends to be a bit more flexible or loose (while still having a structure).

table

index

The target against which queries, whether in SQL or Elasticsearch get executed against.

schema

implicit

In RDBMS, schema is mainly a namespace of tables and typically used as a security boundary. Elasticsearch does not provide an equivalent concept for it. However when security is enabled, Elasticsearch automatically applies the security enforcement so that a role sees only the data it is allowed to (in SQL jargon, its schema).

catalog or database

cluster instance

In SQL, catalog or database are used interchangeably and represent a set of schemas that is, a number of tables.
In Elasticsearch the set of indices available are grouped in a cluster. The semantics also differ a bit; a database is essentially yet another namespace (which can have some implications on the way data is stored) while an Elasticsearch cluster is a runtime instance, or rather a set of at least one Elasticsearch instance (typically running distributed).
In practice this means that while in SQL one can potentially have multiple catalogs inside an instance, in Elasticsearch one is restricted to only one.

cluster

cluster (federated)

Traditionally in SQL, cluster refers to a single RDMBS instance which contains a number of catalogs or databases (see above). The same word can be reused inside Elasticsearch as well however its semantic clarified a bit.

While RDBMS tend to have only one running instance, on a single machine (not distributed), Elasticsearch goes the opposite way and by default, is distributed and multi-instance.

Further more, an Elasticsearch cluster can be connected to other clusters in a federated fashion thus cluster means:

single cluster::
Multiple Elasticsearch instances typically distributed across machines, running within the same namespace.
multiple clusters::
Multiple clusters, each with its own namespace, connected to each other in a federated setup (see Cross cluster Search).

As one can see while the mapping between the concepts are not exactly one to one and the semantics somewhat different, there are more things in common than differences. In fact, thanks to SQL declarative nature, many concepts can move across Elasticsearch transparently and the terminology of the two likely to be used interchangeably through-out the rest of the material.